Weird font in Firefox

I'm having this problem where my fonts in Firefox changed to this weird ugly bold one and I can't seem to change it. My font settings are exactly the same as they are on my other computer and that one's fonts are normal. I'm running Windows Vista.

The top picture is the abnormal one and the bottom is a normal picture for comparison.

Font corruption is one possibility. Wrong firefox setting is another. https://support.mozilla.com/en-US/kb/Some%20text%20shows%20up%20bold%20after%20upgrade

Have you checked if the same happens with IE? If yes, some of your fonts are corrupted. You can fix this by extracting the fonts from Windows Vista installation disc (if you have it) or use your favorite search engine and do a look up for Microsoft Core Fonts and Microsoft ClearType Fonts Collection. I assume you already know how to install fonts.

If IE doesn't show the same problem, however, you can first try to change the default font in Firefox ( Tools > Options... > Content > Fonts & Colors > Advanced... ) to any other font, restart, change the font back as before, and restart again. If this doesn't work, you'll have to wipe your Firefox profile, and create a fresh one. See here for more information on that.

Try going to View>Zoom and uncheck "Zoom Text Only". I loaded up FF on a VM, and tested it.